This invention relates to a fluorescent display device, and more particularly to an improvement in a filamentary cathode serving as an electron source for a fluorescent display device.
A filamentary cathode which has been conventionally used as an electron source for a fluorescent display device comprises a heater wire (filament) made of tungsten (W) and covered with an electron emitting layer. The conventional fluorescent display device is so constructed that the filamentary cathode is stretchedly arranged under tension in an envelope while being fixed at both ends thereof on support members by welding.
Unfortunately, the conventional filamentary cathode including the W filament has a disadvantage that an end cool is increased in length. The words "end cool" indicates a portion of each of both ends of a filamentary cathode which is deteriorated in electron emission capability due to an end cooling effect which causes a decrease in temperature by transmission of heat from both ends of the filamentary cathode to the support members. For example, in the case of the W filament of 0.64 MG in diameter (corresponding to 0.64 mg in weight per 20 cm in length), a length of the end cool on each of both ends of the filament is as large as about 8 mm. A ratio of a display area of a fluorescent display device to an outer configuration thereof is reduced because of a thickness of a plate material for the envelope and a space for arrangement of the support members for the filament in the envelope. Also, the ratio is further reduced by formation of the end cool.
In view of the above, it is proposed to stretchedly arrange an auxiliary filament at the end cool to compensate for the deterioration in electron emission capability of the end cool. Unfortunately, such proposal causes a structure of the fluorescent display device to be substantially complicated.
Use of a thin W filament of increased electrical resistance for increasing heat generation permits a length of the end cool to be reduced. However, such a W filament exhibits a disadvantage of being deteriorated in breaking strength and current discharge capability. The W filament is decreased in breaking strength to half at a temperature of about 600.degree. C. which it reaches during driving of the fluorescent display device, so that it is impossible to stretchedly arrange the W filament under increased tension when it is mounted in the fluorescent display device. Unfortunately, this causes the W filament to produce vibration of increased amplitude, leading to flickering in a luminous display.